More Than You Ever Wanted to Know About Epistemic Two-Dimensionalism
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More than you ever wanted to know about epistemic two-dimensionalism phil 93515 Jeff Speaks February 12, 2007 1 From contextual to epistemic two-dimensionalism . 1 2 Epistemic two-dimensionalism and the status of [T1] . 2 2.1 The epistemic understanding of scenarios . 3 2.2 The metaphysical understanding of scenarios . 4 3 Some more detail about epistemic intensions . 5 3.1 Canonical descriptions . 5 3.2 The epistemic intensions of names . 6 3.3 Epistemic intensions and definite descriptions . 7 4 Potential explanatory roles for epistemic intensions . 8 4.1 Explaining a priori knowledge . 8 4.2 Solving Frege’s puzzle . 8 4.3 Playing a role in the semantics of attitude ascriptions . 9 4.4 Giving a level of mental content which can explain behavior . 10 4.5 Providing a link between conceivability and possibility . 10 5 Objections to epistemic two-dimensionalism . 11 5.1 Problems with contextual two-dimensionalism revisited . 11 5.2 Consequences of denying supervenience without a priori deducibility . 12 5.3 The nature of the ‘possibilities’ ruled out by de re assertions . 12 5.4 Contents vs. intensions . 13 5.5 A posteriori necessities in semantically neutral vocabulary . 13 1 From contextual to epistemic two-dimensionalism In response to the problems with the two dimensionalist system which (I suggested) is found in The Conscious Mind, Chalmers has modified his views. The main change is that he now rejects his earlier close association of primary intensions with Kaplan- style characters, and consequently also rejects the view that a sentence is a priori iff it is true whenever it is uttered. Because the earlier version of two-dimensionalism took primary intensions to be functions from contexts to extensions, he calls this contextual two- dimensionalism. Because the later version of two-dimensionalism takes primary intensions to be functions from epistemic possibilities to extensions, he calls this the epistemic version of two-dimensionalism. To keep things straight: primary intension = epistemic intension scenario = epistemic possibility = the kind of thing which is argument for an epistemic intension secondary intension = subjunctive intension So we hold in place the two central theses of The Conscious Mind: [T1] A sentence is a priori iff it has a necessary primary inten- sion. [T2] A sentence is (metaphysically) necessary iff it has a neces- sary secondary intension. But we now reject the third: [T3] Truth conditions for primary intensions can be defined in terms of Kaplan’s characters. A sentence’s primary inten- sion is true at a world w iff relative to w as context, the sentence delivers a secondary intension which is true at w. And replace it with: [T3*] Primary intensions are epistemic intensions. Epistemic in- tensions are functions from epistemic possibilities — hy- potheses about the world that cannot be ruled out a priori — to extensions. The primary intension of S is true at a scenario e iff e epistemically necessitates S. Combining [T3*] rather than [T3] with [T1], we get that a sentence is a priori iff every epistemic possibility epistemically necessitates the sentence. 2 Epistemic two-dimensionalism and the status of [T1] The key question about the framework concerns the nature of epistemic possibilities — the things which epistemic intensions take as arguments, and deliver extensions as values — which Chalmers calls ‘scenarios.’ Regarding scenarios, we have two possibilities: (i) define them in epistemic terms, taking epistemic possibility as a primitive, or (ii) define them in terms of centered possible worlds (Chalmers (2006a), §3.4). 2 2.1 The epistemic understanding of scenarios Here we consider the first possibility, which we can call the epistemic understanding of scenarios: [T4] e is a scenario ≡df e is an epistemically possible and com- plete hypothesis about the world that cannot be ruled out a priori. Think of these maximal hypotheses of being long sentences of an ideally expressive lan- guage that are both epistemically possible (not ruled out a priori) and epistemically complete (such that there is no sentence S which is such that both it and its negation can be conjoined to the scenario to yield an epistemically possible sentence). If the language is indeed ideally expressive, then we can assume the following Epistemic Plenitude principle: [EP] Every epistemic possibility is epistemically necessitated by some epistemically possible and complete hypothesis. Let’s add to this the view that epistemic necessitation is a priori entailment, so that S epistemically necessitates e iff the material conditional (S → e is a priori). This gives us a definition of truth at a world for primary intensions: [T5] e epistemically necessitates S ≡df (e → S) is a priori. Recall our discussion of contextual two-dimensionalism. There we had an understanding of what primary intensions were supposed to be which was independent of [T1], and we could go on to ask whether we could assign primary intensions to expressions in some way which would make [T1] come out true. This turned out to be difficult to do. By contrast, now there is no problem at all in reconciling the present view of primary intensions with [T1]. After all, [T4], [T5], and [EP] jointly entail [T1]. To see that, consider the following argument by cases (the connection between primary and epistemic intensions is taken for granted): Suppose that S is a priori. S is true at every scenario iff for any scenario e, e epistemically necessitates S iff for any scenario e, the conditional (e → S) is a priori. But if S is a priori, then any conditional with S as consequent will be a priori. So S is true at every scenario. Suppose now that S is not a priori. Then the negation of S is epistemically possible. So, by [EP], the negation of S is epistemically necessitated, and therefore true at, some scenario w. But scenarios are always epistemically possible, so no scenarios contain both S and its negation. So S is false at w. This gives us both directions of [T1]. However, it does not give us both directions of [T1] in an especially impressive way. Epistemic intensions are simply defined in such a way that [T1] comes out true. Anyone who grants that epistemically possible and complete sentences make sense, and that some conditionals are a priori knowable, should therefore concede that, on this understanding of primary intensions, every a priori truth has a necessary primary intension. 3 Does this mean that the two-dimensionalist has won the day? No. All that has been shown so far is that we can define a property of sentences (the property of having a necessary epistemic intension) such that a sentence has it iff it is a priori. Given that this property was defined using the property of a priority, this by itself does not show anything. Chalmers (2006a) (§3.11) recognizes that [T1] is simply made true by definition on this approach. But, he says, this does not make two-dimensionalism trivial. Rather, the value of epistemic two-dimensionalism consists in its giving us a well-defined value of expressions which can be used to explain various phenomena. The value of the system consists in how well it explains those phenomena. We will return at the end to the question of how well epistemic intensions, on the present interpretation, can explain the phenomena in question. 2.2 The metaphysical understanding of scenarios First we turn to a different understanding of scenarios, which does not seem to make [T1] true by definition. This is what Chalmers (2006a), §3.4 calls the metaphysical understand- ing of scenarios. [T4*] e is a scenario ≡df e is a centered possible world (a meta- physically possible world with a designated agent and time). We then need to replace Epistemic Plenitude with Metaphysical Plenitude: [MP] Every epistemic possibility is epistemically necessitated by some centered metaphysically possible world. [MP] should not be understood to entail the false thesis that ‘It is metaphysically possible that water is not H2O’ is true. On the present view what is required is not that there be possible world with respect to which what we say by ‘Water is not H2O’ is true, but only one which is such that, were that world actual, and were we to know everything about it, it would be a priori that ‘Water is not H2O’ is true. We keep for now the definition of epistemic necessitation in terms of the a priori given by [T5]. It is clear that just as [T4], [T5], and [EP] imply [T1], so also [T4*], [T5], and [MP] imply [T1], so long as we assume one further plausible thesis: [T6] Every centered metaphysically possible world is also epis- temically possible. We can then argue as follows: Suppose that S is a priori. S is true at every scenario iff for any scenario e, e epistemically necessitated S iff for any scenario e, the conditional (e → S) is a priori. But if S is a priori, then any conditional with S as consequent will be a priori. So S is true at every scenario. 4 Suppose now that S is not a priori. Then the negation of S is epistemically possible. So, by [MP], the negation of S is epistemically necessitated, and therefore true at, some scenario w. But by [T4*] and [T6] scenarios are always epistemically possible, so no scenarios contain both S and its negation. So S is false at w. Again, we pretty much have the ‘core thesis’ of two-dimensionalism, [T1], entailed by the definitions of primary intension, scenarios, and epistemic necessitation. The only difference between this case and the case of the epistemic understanding of scenarios is that here we also have to assume [T6], which is plausible, and [MP], which is less clearly true.